Technical Field
The present invention relates to the field of membrane reactors and more specifically membrane reactors having a silica outer layer, a γ-Al2O3 intermediate layer, and a base supporting substrate of α-Al2O3 substrate. The membrane reactor has gas separation properties upon a hydrothermal treatment.
Description of the Related Art
The “background” description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description which may not otherwise qualify as prior art at the time of filing, are neither expressly or impliedly admitted as prior art against the present invention.
It is well established that inorganic membranes are superior to organic membranes for separation processes at high temperatures and pressures, when frequent cleaning is required, and for aggressive chemical, mechanical, or microbiological mixture separations and/or purifications. At least seven-metastable structures including the γ, η, δ, θ, κ, and χ alumina's, as well as its stable α-alumina phase have been reported. See T. M. H. Costa, M. R. Gallas, E. V. Benvenutti, J. A. H. da Jornada, Study of Nanocrystalline γ-Al2O3 Produced by High-Pressure Compaction. J. Phys. Chem. B 103 (1999) 4278-4284, incorporated herein by reference in its entirety. Of these, γ-Al2O3 has been utilized in many applications, in the form of powder or thin film, as catalysts or catalyst carriers, coating, adsorbents, as well as catalyst supports for cracking, hydrocracking, and hydrodesulfurization of petroleum feed stocks. See K. Wefers, Alumina Chemicals: Science and Technology Handbook (Ed.: L. D. Hart), The American Ceramic Society, Westerville, Ohio, (1990) p. 13, incorporated herein by reference in its entirety. All these processes require high temperatures; water vapor (steam) is usually one of the reactants, or sometimes steam is added to these systems to reduce coke formation. See V. Boffa, D. H. A. Blank, J. E. ten Elshof, Hydrothermal Stability of Microporous Silica and Niobia-Silica Membranes. J. Membr. Sci. 319 (2008) 256-63, incorporated herein by reference in its entirety. Thus, γ-Al2O3 catalysts and/or membranes have to withstand the presence of steam and high temperatures. However, very few studies have examined the hydrothermal stability of γ-Al2O3, in particular γ-Al2O3 membranes on α-Al2O3 supports.
The design of asymmetrically fabricated membranes is highly advantageous for gas separation and water purification applications. This type of membrane can provide high selectivity and acceptable permeability. See R. M. de Vos, H. Verweij, High-Selectivity, High-Flux Silica Membranes for Gas Separation, Science, 279 (1998) 1710-1711, incorporated herein by reference in its entirety. The porous structure of such membranes varies continuously within the membrane thickness according to the variations in the pore structure of the support and the boehmite (γ-AlOOH) sol concentration. Therefore, membrane morphology and pore structure are crucial to the fabrication of highly selective gas separation membranes especially supported membranes in the presence of steam. State-of-the-art microporous silica membranes can separate hydrogen from other gas molecules. In gas separation membrane fabrication, a mesoporous (2 nm<Ø<50 nm) γ-Al2O3 membrane is placed between a layer consisting of a silica membrane on the top and an α-Al2O3 porous support on the bottom. The γ-Al2O3 layer backing provides mechanical strength to the silica top layer. See Md. H. Zahir, K. Sato, Y. Iwamoto, Development of Hydrothermally Stable SolGel Derived La2O3-Doped Ga2O3—Al2O3 Composite Mesoporous Membrane. J. Membr. Sci. 247 (2005) 95-101; Md. H. Zahir, T. Nagano, High-Selectivity Y2O3-Doped SiO2 Nanocomposite Membranes for Gas Separation in Steam at High Temperatures. J. Am. Ceram. Soc. 99 (2015) 1-9, each incorporated herein by reference in their entirety. It has also been reported that α-Al2O3-supported γ-Al2O3 membranes can be used in gas separation. See F. M. Leenaars, K. Keizer, A. J. Burggraaf, The preparation and characterization of alumina membranes with ultra-fine pores, Part 1 Microstructural investigations on non-supported membranes. J. Mate. Science. 19 (1984) 1077-1088, incorporated herein by reference in its entirety. However, in contrast to the advances made in silica membrane technologies, negligible improvement has been made in the performance of mesostructured γ-Al2O3 membranes.
The cross-sectional examination of γ-Al2O3 membranes with sophisticated TEM techniques is important to determine the particle arrangement, which is an important feature of the PSD. See T. Z. Ren, Z. Y. Yuan, B. L. Su, Microwave-Assisted Preparation of Hierarchical Mesoporous-Macroporous Boehmite AlOOH and γ-Al2O3. Langmuir 20 (2004) 1531-1534, incorporated herein by reference in its entirety. Note that the accurate and realistic characterization of supported membranes is crucial because the morphologies of their pore spaces and particle arrangements strongly affect their flow, transport, reaction, and gas separation properties. See Md. H. Zahir, Y. H. Ikuhara, S. Fujisaki, K. Sato, T. Nagano, Y. Iwamoto, Preparation and characterization of mesoporous ceria-zirconia-alumina nanocomposite with high hydrothermal stability. J. Mater. Res. 22 (2007) 3201-3209; Z. Zhaorong, R. W. Hicks, T. R. Pauly, T. J. Pinnavaia, Mesostructured Forms of γ-Al2O3. J. Am. Chem. Soc. 124 (2002) 1592-1593, each incorporated herein by reference in their entirety. The difficulty of characterizing the microstructures of γ-Al2O3 thin films and supported γ-Al2O3 membranes might be due to the problem of differentiating active pores (membrane) and passive pores (support). See R. Mourhatch, T. T. Tsotsis, M. Sahimi, Determination of the true pore size distribution by flow permporometry experiments: An invasion percolation model. J. Membr. Sci. 367 (2011) 55-62, incorporated herein by reference in its entirety. In addition, the delamination of γ-alumina membranes from α-Al2O3 supports when exposed to steam or the cracking and peeling-off of the membrane during drying might occur during cross-sectional morphology investigations. It has been claimed that the N2 adsorption-desorption results for non-supported materials, and their microstructural characteristics, are similar to those of supported thin films. The microstructures of γ-Al2O3 membranes have been studied by using non-supported films, i.e., by performing powder characterization. It has also been assumed that the characteristics of supported and non-supported membranes are comparable, i.e., the results for non-supported films are expected to be applicable to supported films. See R. J. R. Uhlhorn, M. H. B. J. H. In'tveld, K. Keizer, A. J. Burggraaf, Synthesis of ceramic membranes Part I Synthesis of non-supported and supported γ-alumina membranes without defects. J. Mater. Sci. 27 (1992) 527-537, incorporated herein by reference in its entirety. Vos et al. determined the PSDs of unsupported materials by using the Horrath-Kawazoe method and compared these results with those for supported materials. See R. M. de Vos, Henk Verweij, Improved performance of silica membranes for gas separation, J. Membr. Sci. 143 (1998) 37-51, incorporated herein by reference in its entirety. However, Cao et al. and Cuperus et al. introduced the nano-permporometry technique in 1993, which can be used to characterize the active pores of the top separation layer (membrane) only. See G. Z. Cao, J. Meijernik, H. W. Brinkman, A. J. Burggraaf, Permporometry study on the size distribution of active pores in porous ceramic membranes, J. Membr. Sci. 83 (1993) 221-235; F. P. Cuperus, D. Bargeman C. A. Smolders, Permporometry. The determination of the size distribution of active pores in UF membranes, J. Membr. Sci. 71 (1992) 57-67, each incorporated herein by reference in their entirety. The nitrogen adsorption-desorption technique is sensitive to both the active and the passive pores. Therefore, the pore size distributions of supported and non-supported membranes are not comparable.
The interpretation of the existing morphological image data for γ-Al2O3 powders, particularly γ-Al2O3-supported micro- or meso-porous powders or membranes, is challenging. Most micro-level characterization of γ-Al2O3 is not systematic, with only very unclear TEM images and very dark and obscure images of color contrast and/or structure available in the open literature. See G. R. Gallaher, P. K. T. Liu, Characterization of ceramic membranes, I. Thermal and hydrothermal stabilities of commercial 40 Å, membranes, J. Membr. Sci. 92 (1994) 29-44; S. J. Wilson, J. D. C. McConnell, M. H. Stacey, Energetics of formation of lamellar porous microstructures in γ-Al2O3. J. Mater. Sci. 15 (1980) 3081-3090; X. Yang, A. C. Pierre D. R. Uhlmann, Tem study of boehmite gels and their transformation to α-alumina. J. Non-Cryst. Sol. 100 (1988) 371-377, each incorporated herein by reference in their entirety. For example, the TEM results for boehmite of Saraswati et al. and Leenaars et al. are not identical: Saraswati et al. observed rod-like particles whereas Leenaars et al. observed spherical particles. See V. Saraswati, G. V. N Rao, G. V. R. Rao, Structural evolution in alumina gel, J. Mater. Sci. 22 (1987) 2529-2534, incorporated herein in its entirety. A boehmite sol with plate-shaped crystallites has been also reported. Liu et al. and Tijburg et al. provided very clear TEM images of boehmite that are however very different from those provided by Rao and Leenaars et al. See Q. Liu A. Wang X. Wang, T. Zhang, Mesoporous γ-alumina synthesized by hydro-carboxylic acid as structure-directing agent. Micro. Meso. Mater. 92 (2006) 10-21; I. M. Tijburg, H. D. Bruin, P. A. Elberse, J. W. Geus, Sintering of pseudo-boehmite and γ-Al2O3. J. Mater. Sci. 26 (1991) 5945 5949, each incorporated herein by reference in their entirety. Liu did not fabricate supported γ-Al2O3 membranes and thus did not present cross-sectional membrane morphology images. Gallaher et al. presented very unclear TEM images in which nothing was visible. The TEM images provided by Wilson et al. were also very dark, making it hard to examine particle morphologies. Recently, Gaber et al. presented TEM images of γ-Al2O3, but these images are very unclear and appear to show very thick acicular-type particles. See A. A. A. Gaber, D. M. Ibrahim, F. F. A. AImohsen, E. M. El-Zanati, Synthesis of alumina, titania, and alumina-titania hydrophobic membranes via solgel polymeric route. J. Anal. Sci. Tech. 4 (2013) 1-20, incorporated herein by reference in its entirety.
Many groups have reported the thermal stability of γ-Al2O3 by using thermal stabilizing modifiers such as Zr4+, Ca2+. Th4+, and La3+. Of these, La3+ is quite effective, opposite trend could be obtained by using In3+, Ga3+, and Mg2+ metal ions. See M. Trueba, S. P. Trasatti, γ-Alumina as a Support for Catalysts: A Review of Fundamental Aspects, Eur. J. Inorg. Chem. (2005) 3393-3403, incorporated herein by reference in its entirety. Zahir et al. have investigated the effects on the thermal and hydrothermal stability of γ-Al2O3 membranes of the addition of La2O3 prepared with the sol-gel method.
In view of the forgoing, the objective of the present disclosure is to present a semi-porous composite membrane and a method of making thereof.